Pultrusion of fiber reinforced composite materials has been used to produce constant cross-sectional hollow structures. However, many structures using hollow members require that they have varying cross-sectional geometry as well as part curvature, twists and joints. Such hollow members may, for example, involve automotive space frames, aircraft hulls and the like.
To date such hollow members or structures have typically been fabricated using a core material as the inner mold. A lightweight urethane foam is generally machined or cast to the correct geometry if the core is to be left inside the structure formed. The core may be dissolved or melted after the hollow structure is cured if the core is to be removed.
When such a core is used, the composite material is either applied wet, preimpregnated with resin, or as a dry reinforcement. When a dry reinforcement composite material is used, resin is subsequently pumped into the material after closing of the mold.
In some cases, inflatable molds have been used to cure outer skins for helicopter rotor blades. In these cases, preimpregnated fibrous reinforcemnt was involved and the inflatable mold had to be carefully fabricated to a specific size and shape.